1. Field of the Invention
This invention relates to impact printers. More specifically, it relates to a ribbon drive mechanism which is utilizable for such impact printers.
2. Description of Prior Art
With the development of the printer field in the direction of high speed impact printers producing high quality printing suitable for correspondence at high speed in the order of 60 cycles per second, new needs have arisen with respect to printer ribbon structures and drive mechanisms.
Because of the high throughput of such printer apparatus and the consequently high volume of printed characters, the art has had to provide a ribbon which is of relatively low cost but yet provides high quality printing. Because of the difficulty in meeting these requirements with the more traditional fabric base or carbon film ribbons, the art has been working with a more recent type of ribbon which is a case matrix of a plastic such as nylon containing liquid ink. While such ribbon structures appear to provide the combination of high quality printing and low cost, they are highly distortable and fragile. Further, they are sensitive to high temperature ad high humidity. For example, at temperatures in the order of 25.degree. C. and 80% relative humidity, as little as 30 grams of ribbon tension may cause objectionable yielding and frequent breakage of a cast matrix type of ribbon which is in the order of 0.6 cm. wide.
These problems become particularly acute in high speed "daisy wheel" printers. Designers of such impact printers have been attempting to minimize the flight path of the selected character petal, i.e., the distance the petal must travel in driving the ribbon against the sheet in order to maximize the speed of the printers. In this connection, distances in the order of a tenth of an inch between the face of the printer wheel petal and the platen would not be unreasonable in high speed printers. Since the ribbon, the cardholder, and the paper all must fit and be translationally movable within this limited space, problems of maintaining very close tolerances between these elements are presented. Because of these close tolerances, the ribbon must be kept at a very shallow angle to the paper surface. Thus, the ribbon tension required to release the ribbon from the paper following impact is greater than that encountered in conventional typeball machines having higher allowable peel angles. In addition, in printers employing "daisy wheels", the physical arrangement of components is such that there is a longer expanse (print span) of unsupported ribbon along the printing surface. This gives rise to greater ribbon tension requirements.
Thus, in implementing such highly fragile and easily distortable ribbon to the requirement of the high speed impact printing art, substantial problems were involved. Such problems included how to maintain a constant and uniform tension required because of the close tolerances in the impact areas and the tensions required to keep the ribbon relatively taut adjacent to the daisy wheel while at the same time driving without substantially distorting or breaking the highly distortable fragile ribbon.